Dirac structure explained

In mathematics a Dirac structure is a geometric structure generalizing both symplectic structures and Poisson structures, and having several applications to mechanics. It is based on the notion of the Dirac bracket constraint introduced by Paul Dirac and was first introduced by Ted Courant and Alan Weinstein.

Linear Dirac structures

Let

V be a real vector space, and V ^{*} its dual. A (linear) Dirac structure on V is a linear subspace D of V x V ^{*} satisfying

• for all

(v,\alpha)\in D one has \left\langle\alpha,v\right\rangle =0 ,

D is maximal with respect to this property.In particular, if V is finite dimensional, then the second criterion is satisfied if \dim D = \dim V . Similar definitions can be made for vector spaces over other fields.

An alternative (equivalent) definition often used is that

D satisfies D = D ^{\perp} , where orthogonality is with respect to the symmetric bilinear form on V x V ^{*} given by l \langle(u,\alpha),(v,\beta) r \rangle = \left\langle\alpha,v\right\rangle + \left\langle\beta,u\right\rangle .

Examples

1. If
   U\subset V is a vector subspace, then D = U x U ^{\circ} is a Dirac structure on V , where U ^{\circ} is the annihilator of U ; that is, U ^{\circ=\left\{\alpha\in} V ^{*\mid} \alpha _{\vert} =0 \right\} .
2. Let
   \omega:V\to V ^{*} be a skew-symmetric linear map, then the graph of \omega is a Dirac structure.
3. Similarly, if \Pi:V ^{*\to} V is a skew-symmetric linear map, then its graph is a Dirac structure.

Dirac structures on manifolds

A Dirac structure

ak{D} on a smooth manifold M is an assignment of a (linear) Dirac structure on the tangent space to M at m , for each m \in M . That is,

• for each

m\in M , a Dirac subspace D _{m} of the space T _{mM x} T ^{*} _{mM} .Many authors, in particular in geometry rather than the mechanics applications, require a Dirac structure to satisfy an extra integrability condition as follows:

• suppose

(X _{i,\alpha} _{i)} are sections of the Dirac bundle ak{D} \to M (i =1 , 2 , 3 ) then \left\langle L _{X _{1}} (\alpha _{2),X} _{3\right\rangle +} \left\langle L _{X _{2}} (\alpha _{3),X} _{1\right\rangle +\left\langle} L _{X _{3}} (\alpha _{1),X} _{2\right\rangle} = 0 .

In the mechanics literature this would be called a closed or integrable Dirac structure.

Examples

1. Let
   \Delta be a smooth distribution of constant rank on a manifold M , and for each m\in M let D _{m=\{(u,\alpha)\in} T _{mM x} T ^{*M} _{m} \mid u\in\Delta(m),\alpha\in \Delta(m) ^{\circ\}} , then the union of these subspaces over m forms a Dirac structure on M .
2. Let \omega be a symplectic form on a manifold M , then its graph is a (closed) Dirac structure. More generally, this is true for any closed 2-form. If the 2-form is not closed, then the resulting Dirac structure is not closed.
3. Let \Pi be a Poisson structure on a manifold M , then its graph is a (closed) Dirac structure.

Applications

Thermodynamics

References

• H. Bursztyn, A brief introduction to Dirac manifolds. Geometric and topological methods for quantum field theory, 4–38, Cambridge Univ. Press, Cambridge, 2013.
• Henrique . Bursztyn . Marius . Crainic . 2005 . Dirac structures, momentum maps, and quasi-Poisson manifolds. The Breadth of Symplectic and Poisson Geometry . Progress in Mathematics. 232 . Birkhauser-Verlag. 1–40.
• Theodore . Courant . Dirac manifolds. Theodore Courant . Transactions of the American Mathematical Society. 319 . 1990 . 2 . 631–661. 10.1090/S0002-9947-1990-0998124-1 . free .
• Theodore . Courant . Alan . Weinstein. Theodore Courant . Alan Weinstein . Beyond Poisson structures. Séminaire sud-rhodanien de géométrie VIII. Travaux en Cours . 27 . Paris . Hermann. 1988.
• Book: Dorfman, Irène . 1993 . Dirac structures and integrability of nonlinear evolution equations . Wiley.
• François . Gay-Balmaz . Hiroaki . Yoshimura . 2020 . Dirac structures in nonequilibrium thermodynamics for simple open systems . . 61 . 9 . 092701 (45 pp). 10.1063/1.5120390 . 1907.13211 . 2020JMP....61i2701G . 199001204 .
• Arjan . van der Schaft . Bernhard M. . Maschke . Arjan van der Schaft . 2002 . Hamiltonian formulation of distributed-parameter systems with boundary energy flow . . 42 . 1–2 . 166–194. 10.1016/S0393-0440(01)00083-3 . 2002JGP....42..166V .
• Hiroaki . Yoshimura . Jerrold E. . Marsden . Jerrold Marsden . 2006 . Dirac structures in Lagrangian mechanics. I. Implicit Lagrangian systems . . 57 . 133–156. 10.1016/j.geomphys.2006.02.009 .
• Hiroaki . Yoshimura . Jerrold E. . Marsden . Jerrold Marsden . 2006 . Dirac structures in Lagrangian mechanics. II. Variational structures . . 57 . 209–250. 10.1016/j.geomphys.2006.02.012 . 10.1.1.570.4792 .